Vehicles for longer travel durations are regularly equipped with lavatory units. For example, aircrafts are provided with lavatory units to provide respective service to passengers. Further examples for lavatory units on-board a vehicle, i.e. vehicles equipped with lavatory units are trains and also coaches. Of course, also ships may be equipped with lavatory units. In particular aircrafts, such as airplanes or helicopters, as well as trains and coaches have space-saving requirements for cabin layout and for all service units to be integrated in the vehicle, because, among other reasons, the space used for service units, such as lavatory units, can no longer be used for providing passenger seats. Thus, lavatory units for use on-board a vehicle should be as small as possible. However, small lavatory units with minimized space relations may not allow to be used by persons with reduced mobility, for example by persons sitting in a wheelchair. Therefore, at least a selection of lavatory units on-board a vehicle must also consider the specific requirements resulting from the use by persons sitting in wheelchairs. For example, WO 2011/101385 A2 describes a toilet assembly for a means of transportation, for example an aircraft. A partition between two adjoining toilet spaces is movably mounted and designed to be moved into an open position, in which a separation between the two toilet spaces is eliminated. In this way, two relatively small toilet spaces can be converted into a larger toilet space, which is suitable for use by persons having limited mobility, for example a person sitting in a wheelchair. However, this solution requires in minimum two toilet spaces at a common location for providing a toilet space that can be used by a person with reduced mobility.